This invention relates to dual chamber cardiac pacemakers and, more particularly, such pacemakers which have means for reprogramming of operating parameters, and particularly the atrial refractory period.
Dual chamber cardiac pacemakers have come into increasing use in recent years. The advantages of dual chamber pacemakers over single chamber pacemakers, i.e., those that pace and/or sense only in the ventricle or the atrium, are known in the art and have been the subject of extensive literature. However, a recurring and longstanding problem with the dual chamber pacemakers is their vulnerability to pacemaker mediated tachycardia which is caused by sensing retrograde P waves and increasing pacing rate as a result thereof. As has been described in the literature, a patient's heart has a retrograde conduction path from the ventricle back to the atrium, even frequently in cases where there is no antegrade conduction. As a result, a delivered ventricular stimulus may be sensed by the atrial lead of a dual chamber pacemaker, and falsely recognized as a natural P wave. When this happens, the pacemaker timing is reset earlier than it should be, i.e, it is reset at the time of the sensed retrograde P wave instead of at the time of occurrence of the next naturally occurring P wave which would have come later. Thus, the succeeding ventricular stimulus is delivered sooner than otherwise, the cycle repeats, and the pacemaker rate goes dangerously high.
There has been a great deal of effort in the pacemaker industry to solve the problem of pacemaker induced tachycardia. Many of these efforts have resulted in the response of attempting to properly set the atrial refractory period, so that the pacemaker does not act upon a P wave which is sensed early and before the naturally occurring P wave is expected. Even in such systems, problems often result. For example, some dual chamber pacemakers attempt to limit the frequency of operation caused by naturally occurring high rate atrial signals, by stretching out the A-V delay, in what is called the Wenckebach mode of operation. As is known in the art, extending the A-V delay increases the vulnerability to retrograde P waves being sensed after time-out of the atrial refractory period. Basically, the problem is that the longer the atrial refractory period, the less is the chance of detecting natural P waves through a rate range that is anticipated; the shorter the atrial refractory period, the more the probability of sensing retrograde P waves. Thus, there may be a problem with setting the atrial refractory period either too long or too short, and this dilemma has plagued the industry.
One approach to the problem that has been embodied in a commercially available pacemaker system is to determine the V-A conduction time at the time when the pacemaker is being implanted, and to set the atrial refractory period so that it extends just safely beyond the time when the retrograde P wave should arrive in the atrium. Since the A-V delay is programmed, if the V-A conduction time is known, it can be determined when in the pacemaker cycle it should be safe to sense atrial signals with assurance that they are natural signals and not retrograde P waves. However, this is not a safe practice for the reason that a patient's V-A conduction time can change and generally does change with time. It is known that both the natural antegrade and retrograde conduction times will change, and frequently do change after pacemaker implantation. This being the case, the determination of retrograde V-A conduction time at the time of implant will have little practical value throughout much of the pacemaker lifetime.
Another known technique in the art is that of delivering a single early stimulus in the ventricle, the timing being designed to produce a retrograde P wave before the next expected natural P wave appears. Measurement of the time interval between the early ventricular stimulus and the following P wave should, if it is a retrograde P, give the V-A conduction time. However, such a "one-time" measurement is very unreliable. There is no assurance that a retrograde P has been detected, and indeed the physician would be uncertain as to whether the patient's heart even exhibited retrograde conduction at that time. Further, since conduction rate varies somewhat with pacing rate, there would be less than an optimal amount of information from which the physician could intelligently choose a refractory period.
There thus remains a substantial need in the industry for a means and method of non-invasively determining a patient's retrograde conduction time, so that the atrial refractory interval can be adjusted accordingly. It is recognized that atrial refractory period is a programmable parameter which, in many existing pacemaker systems can be programmed from external apparatus. This being the case, if a means for non-invasively determining the V-A conduction time can be found, the atrial refractory period can be optimally adjusted. Further, it would be desirable to have automatic means in an implanted pacemaker for determining the V-A conduction time and automatically adjusting the atrial refractory time or pacing mode accordingly.